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restrictive in reality, it makes perfect sense when we think of the purpose of
our system. We are interested in simulating Von Neumann architecture as a
whole, not the single components by themselves.
The computer provides a unitary view of the system. It is used to instan-
tiate the whole system and it represents the interface for starting and con-
trolling the simulation. The computer must provide a method for advancing
the simulation of a given number of steps.
The classes that we use to model the computer architecture are the same as
those identified at the analysis level. The class diagram is shown in
Figure 5.5. The Computer class aggregates the other components, i.e. the CPU ,
the Bus , and the RAM . Both the CPU and the RAM are connected to the Bus .
Decision point
How does the simulation evolve?
The simulation evolves in terms of computer steps. A step corresponds to
an atomic operation, e.g. the CPU reading data from the bus.
We want to be able to specify the number of steps of simulation that must
be executed. Thus, we need a way to trigger a single step of simulation for
each component. The method execute() of CPU and RAM serves this purpose.
Simulation can be performed as shown in the sequence diagram of Figure
5.6. The Computer class provides the user with a single functionality repre-
sented by method simulate() , which starts and controls the simulation.
The user requests the computer to perform one step of simulation, which
determines one step of simulation for each of the components, i.e. the CPU
and the RAM. The order of execution is also important. Since the CPU is the
master of the system it must execute its simulation step before the other
! simulate()
0 bus
0 bus
0 data : String
0 command : String
0 address : int
! execute()
! execute()
! dump()
Figure 5.5 Class diagram of the simulated Von Neumann architecture
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